In Long Term Evolution (LTE) systems, advanced modulation has been introduced for use in downlink transmissions to increase the wireless device's data throughput when channel link quality is good. With the addition of advanced modulation, a downlink transmission from a base station or a network node or an evolved Node B (eNB) to a connected wireless device, such as user equipment (UE), can be configured as either a High Order Modulation (HoM) mode, where the modulation set used is, for example, {Quadrature Phase Shifting Key (QPSK), 16 Quadrature Amplitude Modulation (QAM), 64 QAM, 256 QAM}, or a legacy mode, where the modulation set used can be, for example, {QPSK, 16QAM, 64 QAM}. Depending upon the downlink channel feedback from the UE, the base station, e.g., eNB, can select one of these download transmission modes.
The wireless device can receive the mode selection decision from the base station in a Radio Resource Control (RRC) connection reconfiguration message. Based on the choice of the modulation set, the downlink transmission quality measurements are mapped to the Channel Quality Indicator (CQI) levels by the wireless device and reported back to the serving base station. Similarly, the serving cell or eNB or base station indicates the index of the Modulation and Coding Scheme (MCS) used in the Physical Downlink Shared Channel (PDSCH) transmission as part of the Downlink Control Information (DCI). The MCS index is selected from either the HoM mode MCS set or the legacy mode MCS set.
To maintain the backward compatibility with the legacy wireless devices, the number of MCSs and the CQI levels for both the modulation modes are kept at 32 and 16 respectively. However, the mapping of these to specific modulation and coding schemes and CQI levels depend on the specific modulation set mode selection. The switching mechanism between the modes is left to the network vendor's implementation. The serving cell sends RRC connection reconfiguration messages to the wireless device to inform of the transition from one mode to another. CQI feedback from the wireless device can be used to determine whether or not to select HoM mode. However, merely examining a series of CQI feedback reports e.g., CQI is reported as “15” in a series of feedback reports, from the wireless device is not always effective in determining whether to switch transmission to HoM mode. For example, when the wireless device is operating in one code word transmission, based on CQI feedback, the wireless device may be switched to a single layer HoM mode or a two code word, multi-layer transmission, using the legacy mode.